Light source module, optical scanning device, and image forming apparatus

Abstract

A coupling lens collimates an optical beam emitted from a laser light source. An aperture shields a peripheral light flux area of the collimated optical beam. A phase adjusting element partially changes a phase of wavefront of the optical beam. An auxiliary aperture is formed on an outer side of a normalized aperture size in at least one of a main scanning direction and a sub-scanning direction. The phase adjusting element is formed in a parallel plate, and changes at least a phase of wavefront of a peripheral portion of the optical beam to compensate decreases of the beam spot size and the depth allowance caused by the auxiliary aperture.

1. A light source module for an optical scanning device including a deflecting unit that deflects an optical beam and an imaging optical system that focuses the optical beam on a scanning surface as a beam spot to optically scan the scanning surface, the light source module comprising:
 a laser light source that emits an optical beam; 
 a coupling lens that collimates the optical beam emitted from the laser light source, and outputs a collimated optical beam having a desired beam spot size; 
 an aperture that shields a peripheral light flux area of the collimated optical beam output from the coupling lens; 
 a phase adjusting element that partially changes a phase of wavefront of the optical beam; and 
 an auxiliary aperture formed on an outer side of a normalized aperture size, which is a size of the aperture when the beam spot formed on the scanning surface meets design values of a beam spot size and a depth allowance, in at least one of a main scanning direction and a sub-scanning direction, wherein 
 the phase adjusting element is formed in a parallel plate, and changes at least a phase of wavefront of a peripheral portion of the optical beam to compensate decreases of the beam spot size and the depth allowance caused by the auxiliary aperture. 
 
   
   
     2. The light source module according to  claim 1 , wherein the laser light source is a vertical cavity surface emitting laser including at least one light emitting source. 
   
   
     3. The light source module according to  claim 1 , wherein the phase adjusting element is symmetric about a center of the aperture. 
   
   
     4. The light source module according to  claim 1 , wherein the auxiliary aperture is formed by extending the aperture to the outer side of the normalized aperture size in at least on of the main scanning direction and the sub-scanning direction. 
   
   
     5. The light source module according to  claim 1 , wherein
 the auxiliary aperture includes at least one pair of auxiliary apertures formed separately on the outer side of the normalized aperture size in at least one of the main scanning direction and the sub-scanning direction, and 
 the phase adjusting element changes the phase at portions corresponding to the auxiliary apertures. 
 
   
   
     6. The light source module according to  claim 5 , wherein the phase adjusting element changes the phase such that an average phase in the auxiliary apertures differs from an average phase of the aperture. 
   
   
     7. The light source module according to  claim 1 , wherein the phase adjusting element includes an antireflection unit on at least one of an incident surface and an output surface. 
   
   
     8. An image forming apparatus that forms a color image by forming at least two electrostatic latent images on at least one image carrier using an optical scanning, developing the electrostatic latent images into toner images in different colors, and transferring the toner images onto a transfer medium in a superimposed manner, the image forming apparatus comprising the light source module according to  claim 1  as many as number of the image carriers. 
   
   
     9. An optical scanning device comprising:
 a deflecting unit that deflects an optical beam emitted from at least one laser light source; 
 at least one imaging optical system that focuses the optical beam deflected by the deflecting unit on at least one scanning surface as a beam spot; and 
 at least one light source module including
 a laser light source that emits an optical beam, 
 a coupling lens that collimates the optical beam emitted from the laser light source, and outputs a collimated optical beam having a desired beam spot size, 
 an aperture that shields a peripheral light flux area of the collimated optical beam output from the coupling lens, 
 a phase adjusting element that partially changes a phase of wavefront of the optical beam, and 
 an auxiliary aperture formed on an outer side of a normalized aperture size, which is a size of the aperture when the beam spot formed on the scanning surface meets design values of a beam spot size and a depth allowance, in at least one of a main scanning direction and a sub-scanning direction, wherein 
 
 the phase adjusting element is formed in a parallel plate, and changes at least a phase of wavefront of a peripheral portion of the optical beam to compensate decreases of the beam spot size and the depth allowance caused by the auxiliary aperture. 
 
   
   
     10. The optical scanning device according to  claim 9 , wherein
 the laser light source is a vertical cavity surface emitting laser including at least one light emitting sources, and 
 a following relationship is satisfied
     dm/fm<ds/fs,   
 
  where dm and ds are widths of the aperture including the auxiliary aperture in the main scanning direction and in the sub-scanning direction, respectively, and fm and fs are focal lengths of an optical system at a subsequent stage of the aperture in the main scanning direction and in the sub-scanning direction, respectively. 
 
   
   
     11. The optical scanning device according to  claim 9 , wherein the phase adjusting element changes the phase in the sub-scanning direction only. 
   
   
     12. An image forming apparatus that forms an image by forming an electrostatic latent image on an image carrier using an optical scanning and developing the electrostatic latent image into a toner image, the image forming apparatus comprising the optical scanning device according to  claim 9  to perform the optical scanning.